Several UK airports are closed and train services have been cancelled due to heavy snowfall in many areas.
Police in Northern Ireland - where conditions are said to be the worst in 25 years - and parts of Scotland are warning against non-essential travel.
At least 800 schools in Wales, 500 in Scotland and 700 in Northern Ireland closed.
More snow is forecast later in western areas and on Saturday, 10in (25cm) is possible in Wales and southern England.
The Met Office has issued severe weather warnings for heavy snow - up to 8in - in many places, including Orkney and Shetland, Highlands and Outer Hebrides, Grampian, Northern Ireland, Wales, the Midlands, and the south-east and south-west of England.
Forecasters have warned that in parts of Scotland temperatures could drop to -20C (-4F) overnight where snow is lying.
Strong winds will cause the snow to drift in some areas and ice could be a problem anywhere in the UK, with temperatures struggling to get above freezing.
The BBC Northern Ireland weather centre said the current snow situation was probably the worst widespread fall in about 25 years.
In 2000, similar amounts of snow fell in about four hours, but were restricted to eastern counties.
Drivers in Londonderry, Antrim and Down are being warned only essential travel should be undertaken.
By Saturday, a second band of heavy snow is forecast to affect southern England and Wales, with 2-4in likely in many places and 8-10in possible in some others. Forecasters say there is some uncertainty about how far north it will reach.
Problems caused by the weather include:
Friday was due to be the last day of term in many schools, but hundreds have closed their doors early because of the weather.
New Scottish Transport Minister Keith Brown has insisted Scotland is "very well prepared" and the authorities are doing all they can to keep trunk roads open.
Mr Brown's predecessor Stewart Stevenson resigned after his department was criticised for its handling of the problems caused by the previous bout of severe weather.
The RAC reported breakdown callouts reached a peak of about 2,000 an hour at 1400 GMT, with the busiest areas being Wales, Northern Ireland and north-east England.
And the AA breakdown organisation said it had received 11,000 callouts by 1500 GMT, roughly 1,300 every hour.
Spokesman Gavin Hill-Smith said conditions in some areas were the "worst imaginable, even for experienced drivers", with Aberdeenshire worst hit.
The Association of Train Operating Companies (Atoc) said it would run "ghost trains" to try to stop ice forming on the tracks and had hundreds of ground staff on duty to keep key junctions and points clear of snow.
Energy Minister Charles Hendry warned on Thursday that more bad weather over Christmas could lead to "very serious" shortages of domestic heating oil.
But a spokesman for Prime Minister David Cameron has played down the suggestions, saying: "I don't think that's going to happen at all.
"The position at the moment is that there is sufficient fuel. The problem is getting it around the country."
Heavy snowfall during the past few weeks has caused huge disruption, especially in Scotland and north-east England.
Airports, roads and schools were closed, and companies are now warning of a backlog of deliveries which may not reach customers before Christmas.
The Royal Mail is putting on 7,000 delivery rounds this Sunday in a bid to ease the problem.